drm/radeon/dpm: fix typo in setting smc flag

PPSMC_EXTRAFLAGS_AC2DC_GPIO5_POLARITY_HIGH should be
set in extraFlags, not systemFlags.

Noticed-by: Sylvain BERTRAND <sylware@legeek.net>
Signed-off-by: Alex Deucher <alexander.deucher@amd.com>